The present invention relates to a grinding wheel for grinding edges of eye glasses, comprising a base member and an abrasive coating made of diamond and metal. The invention particularly relates to peripheral wheels, in particular including a peripheral profile.
Due to the manufacturing process the abrasive coating of diamond and metal has a relatively small thickness in the range of about 3 mm or less, wherein the diamond particles have a size of about 250 .mu.m or less.
Grinding eye glass edges encounter substantial problems. The eye glass material is very brittle and is likely to break when very small irregularities are encountered. Furthermore the material is rather thin with little strength against lateral forces. Unbalances of the grinding wheel result in substantial waste.
Still further, small machines having thin spindles are used to work eye glasses. Therefore, the weight of the grinding disk is substantial for those machines.
Peripheral grinding disks are known, comprising a base member and a grinding coating, wherein the base member is provided with a central bore to be mounted on a spindle. This is true for all disk types including those having particular bore profiles.
In prior art grinding disks comprising an abrasive coating of diamond and plastic material base members made of a plastic material including metallic fillers as well as natural fillers and base members of aluminum or steel are known. Grinding disks of this type do not lend themselves to the application above referred to as the resin in the abrasive coating does not provide a binding of sufficient stability suitable for finely grinding an extremely brittle and thin material.
Furthermore, known base members, in particular metallic base members have a substantial weight. Admittedly aluminum has a smaller weight than steel, but the bonding between the abrasive coating and the metallic binder and aluminum encounters problems. The bonding can be satisfying, but does not provide an optimum solution because among other problems the expenditure to make such a bonding is substantial.
According to still further prior art the metal of abrasive coatings made of diamond and metal may be bronze of different types such as pure bronze, silver bronze, cobalt bronze and so on, galvanically deposited nickel or even hard metal or hard alloys.
This results in an excellent abrasive coating of substantial wear resistance even under the influence of pressure and heat. However, the base member of up to 95% of all grinding disks consists of steel or bronze because of the joining operation. The resulting disk is heavy, has a high accuracy of symmetry but does not have dampening abilities and fully transmits any unbalances.
Using a base member of resin including metallic fillers the operation of joining the abrasive coating thereon in particular by a pressing, cementing or shrinking process results in a bonding which is not satisfying with respect to the symmetry i.e. the peripheral accuracy of the disk so that the disk is subjected to vibrations. Furthermore the bad heat conductivity of the resin leads to heating problems, in particular when the coating is worn down to a thickness of some tenths millimeters while the heat generated in grinding must be carried away. The abrasive coating may be not applied by sintering as the base member does not have a suitable strength to withstand the pressure and heat of the sintering process. This is the reason that base members of steel and bronze are primarily used for abrasive coatings of diamond and metal.
Still further, base members carrying abrasive coatings tend to deformations under the application of the grinding pressure and heat which are detrimental to creating fine profiles.
Steel has to be pretreated, for example galvanized which increases costs. The pretreatment is necessary for bonding the bronze to the steel.
Summarizing the prior art disks are not suitable for grinding eye glass edges since they result in high waste rates in transmitting unbalances, stiff structure, dampening inability or even unsymmetrical design.
The aim of the present invention is to provide an improved grinding wheel of the type referred to above exhibiting high precision grinding characteristics with respect to accuracy in particular in working an extremely brittle and thin material under economical conditions.